1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device, which senses whether or not the device is touched and the position of a touched region by recognizing a variation of capacitance according to the touch, and a method for fabricating the same.
2. Discussion of the Related Art
Recently, the field of displays visually expressing electric data signals has been rapidly growing. Accordingly, various flat display devices having excellent characteristics, such as thin profile, light weight, and low power consumption, have been developed and rapidly replaced conventional cathode ray tubes (CRT).
Specifically, the flat display devices include a liquid crystal display device (LCD), a plasma display panel device (PDP), a field emission display device (FED), an electro luminescence display device (ELD). All the above flat display devices include a flat display panel to display an image. The flat display panel consists of a pair of transparent insulating substrates, which are bonded to each other with an intrinsic luminescent or polarizing material layer being interposed therebetween.
A liquid crystal display device displays an image by adjusting the light transmittance of liquid crystals using an electric field. Therefore, the liquid crystal display includes a display panel having liquid crystal cells, a backlight unit to irradiate light onto the display panel, and driving circuits to drive the liquid crystal cells.
The display panel includes a plurality of gate lines and a plurality of data lines, which intersect each other to define a plurality of unit pixel regions. Each pixel region is provided with a thin film transistor array substrate and a color filter substrate, which are disposed to be opposite to each other. Spacers are located between the two substrates to maintain a designated cell gap between the two substrates. The cell gap is filled with liquid crystals.
The thin film transistor array substrate includes the gate lines, data lines, thin film transistors, which are switching elements formed at the intersections of the gate lines and the data lines, pixel electrodes respectively formed in liquid crystal cells and connected to the thin film transistors, and an alignment layer applied to the pixel electrodes. The gate lines and the data lines receive signals from the driving circuits through their respective pad parts.
The thin film transistors supply a pixel voltage signal, supplied from the data lines in response to a scan signal supplied to the gate lines, to the pixel electrodes. The color filter array substrate includes color filters formed in liquid crystal cells, a black matrix to divide the color filters from each other and to reflect external light, a common electrode to supply common voltage to the liquid crystal cells, and an alignment layer applied to the common electrode. The thin film transistor array substrate and the color filter array substrate, which are separately manufactured, are aligned and bonded to each other. Thereafter, liquid crystals fill a gap between the two substrates. Thereafter, the inlet, through the liquid crystals are injected, is sealed.
In the liquid crystal display device, manufactured by the process described above, demand for a touch panel, which recognizes the position of a region touched by a hand or a separate input unit and correspondingly transmits a separate data, has increased. Currently, this touch panel is used in a state in which the touch panel is attached to the external surface of a liquid crystal display device. Thus, an attempt to install the touch panel within a panel in the liquid crystal display device has been made. An example of a liquid crystal display device, in which the above mentioned touch panel is installed to prevent the increase in volume caused by the attachment of a separate touch panel to the external surface of the liquid crystal display device, will be described.
Hereinafter, a related art liquid crystal display device will be described with reference to the accompanying drawings.
FIG. 1 is a schematic circuit diagram illustrating a capacitance-type liquid crystal display device according to the related art. FIG. 2 is a circuit diagram illustrating a capacitance sensor of FIG. 1 and a driving method thereof according to the related art.
As shown in FIGS. 1 and 2, the related art liquid crystal display device includes first and second substrates (not shown), which are disposed to be opposite to each other, a liquid crystal layer (not shown) filling a gap between the first and second substrates, gate lines 11 and data lines 12, which intersect each other on the first substrate to define pixel regions, and thin film transistors TFTs formed at the intersections of the gate lines 11 and the data lines 12. A common electrode CE, to which common voltage Vcom is applied, is formed over the entire surface of the second substrate, and pixel electrodes 13 are formed on the pixel regions on the first substrate.
Here, in order to sense capacitance, a first line 21, a second line 22, and a first reference voltage line RL1 to transmit first reference voltage Vref1, and a second reference voltage line RL2 to transmit second reference voltage Vref2 are formed at the outside of the pixel regions. Here, the first line 21 and the first reference voltage line RL1 are arranged to be parallel to the gate lines 11, and the second line 22 and the second reference voltage line RL2 are arranged to be parallel to the data lines 12.
The related art liquid crystal display device further includes first auxiliary capacitors Cref1 formed between the first reference voltage line RL1 and the first line 21, and first liquid crystal capacitors Clc1 formed between the first line 21 and the common electrode CE. The first auxiliary capacitors Cref1 and the first liquid crystal capacitors Clc1 are connected to each other in series. One pair of the first auxiliary capacitors Cref1 and the first liquid crystal capacitors Clc1, which are connected to each other in series, is formed to correspond to each respective pixel region.
The related art liquid crystal display device further includes second auxiliary capacitors Cref2 formed between the second reference voltage line RL2 and the second line 22, and second liquid crystal capacitors Clc2 formed between the common electrode CE and the second line 22. The second auxiliary capacitors Cref2 and the second liquid crystal capacitors Clc2 are connected to each other in series. One pair of the second auxiliary capacitors Cref2 and the second liquid crystal capacitors Clc2, which are connected to each other in series, is formed to correspond to each respective pixel region.
As shown in FIG. 2, a signal sensed by the first line 21 is amplified by an amplifier 31 provided at the end of the first line 21. In other words, the signal sensed by the first line 21 is applied to a node n1 between the auxiliary capacitor Cref and the liquid crystal capacitor Clc, and the amplifier 31 amplifies voltage Vn1 applied to the node n1 to output the output voltage Vout. Whether or not the device is touched and the position of a touched region are determined by the value of the output voltage Vout. In other words, if there is a touch onto the touch panel from the outside, the value of the liquid crystal capacitors Clc corresponding to the position of a touched region is varied, leading to a variation in the value of the voltage Vn1 of the corresponding node n1. Accordingly, the value of the output voltage Vout output through the amplifier 31 changes from the value of the output voltage Vout if there is no touch. Whether or not the device is touched and the position of the touched region are determined based on the above difference of the values of the output voltage Vout.
First and second switches sw1 and sw2 are provided at the opposite side to the output side of the node n1 between the auxiliary capacitor Cref and the liquid crystal capacitor Clc, so that signals can be selectively applied through the first and second switches sw1 and sw2. Two voltage values Vcomh and Vcoml are alternatively applied to the first and second reference voltage lines RL1 and RL2 connected to sides of the first and second auxiliary capacitors Cref1 and Cref2. Voltage VA is applied through the first switch sw1 and stored in the liquid crystal capacitor Clc, when Vcomh is applied, and then is output to the amplifier 31, when Vcoml is applied. Consequently, the output voltage Vout contains data regarding the value of the liquid crystal capacitor Clc, which is varied when touched. A variation in the output voltage Vout according to a variation in capacitance is as follows.
            ∂              V                  n          ⁢                                          ⁢          1                            ∂              C        LC              =            -                        C          ref                                      (                                          C                ref                            +                              C                LC                                      )                    2                      ·          (                        V          comH                -                  V          comL                    )      
The above related art liquid crystal display device, which recognizes a touch using a capacitance method, has several problems. First, a variation in voltage at a point corresponding to one pixel region is selectively sensed to detect whether or not the pixel is touched. Therefore, when several points are touched, it is impossible to recognize whether or not several pixels corresponding to the points are touched. Second, since a touch is sensed through positions of a touched point on the X-axis and the Y-axis, lines intersecting each other in X-axis and Y-axis directions are formed. Therefore, an increase in size of the panel is expected. Further, line resistance of the lines and parasitic capacitance between the lines increase due to the increase in size of the panel. In addition, coupling capacitance increases resulting in a decrease of the signal to noise (S/N) ratio. Accordingly, the reliability of signal detection decreases and thus, it may be difficult to recognize the touch.